1. Field of the Invention
The present invention relates to an apparatus for processing at least one material, and is particularly suited for mixing, finishing and de-volatizing polymers.
2. Description of the Related Art
The commercial preparation of most linear condensation polymers, such as polyamides or polyesters, typically involves heating monomeric starting materials to cause progressive condensation of the polymers. This process is usually carried out in several stages, with the intermediate formation of low-molecular weight, low viscosity polymeric liquid by the removal of volatiles. The low-molecular weight, low-viscosity polymeric liquid then passes through a finishing zone which is controlled at various vacuum and residence times and temperatures to allow the polymer to reach the desired final molecular weight and viscosity.
Undesirable side reactions, such as thermal degradation and discoloration of polymers in polymerization equipment can occur in the preparation of polymers. Such side reactions have long been recognized in the field of polymer processing. Finishers such as those described in U.S. Pat. No. 3,361,537 to Ferrante and U.S. Pat. No. 4,134,736 to Hammond have produced polymers of uniform viscosity, with less thermal degradation, discoloration and gel build-up than finishers which had been previously available.
Separators have also been used in the art of polymer processing for producing polymers of lower molecular weight than finishers produce. Separators are devices which operate at one atmosphere, while finishers operate at one atmosphere or lower with vacuum. It is known to operate a polymerization device as both a separator and a finisher, as exemplifed by U.S. Pat. No. 3,717,330 to Pinhey. It is also known in the art to provide a separator which comprises a spiral ribbon extending upwardly from a transfer screw and a ring disposed above the spiral ribbon, as described in U.S. Pat. No. 3,087,435 to Boucher, with baffles which are connected to a plurality of spokes attached to the transfer screw and disposed below the ring. These baffles have a helical shape and have holes formed therein, and always pump downwardly. The use of a baffle in a reactor, or finisher, is also disclosed in U.S. Pat. No. 4,460,278 to Matsubara et al., U.S. Pat. No. 4,007,016 to Weber, U.S. Pat. No. 3,822,999 to Pope and U.S. Pat. No. 2,804,379 to Wistrich et al.
A major design consideration in polymerizing equipment is flow pattern, which influences final polymer color and quality. Areas of stagnation or channeling are known flow problems and are indicative of non-ideal flow. Ideally, the polymer should flow through a series of several well-mixed stages throughout its residence time in the equipment, while maintaining plug-flow, i.e., substantially uniform liquid flow velocities of all the polymer in a given transverse cross-section of the equipment. The separators and finishers described above do not provide uniform mixing and ideal flow conditions. Also, these finishers and separators exhibit significant gel formation above the surface of the polymer and thermal degradation of the polymer.
As demand increases for polymers of higher molecular weight and for more flexible polymer producing operations with higher throughputs, neither the separators nor the finishers as described above are able to produce desired polymers. With such desired higher molecular-weight polymers and higher throughputs, gel, color and other polymer quality specifications such as thermal degradation index and oxidative degradation index become an increasing problem. To achieve the increased throughput, larger finishers or separators are required. To achieve higher molecular weight, higher viscosity polymer must be processed. The larger finishers or separators and the higher-viscosity polymers require increased power, which results in increased energy input. This increased energy requirement is obviously undesirable from an economic standpoint, and has the additional disadvantages of requiring stronger structures for the finishers or separators and overheating the polymer in the finisher, ultimately resulting in a thermally degraded, inferior product.